Question: Which of these numbers is prime? ${11,\ 25,\ 39,\ 55,\ 76}$
A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 25 are 1, 5, and 25. The factors of 39 are 1, 3, 13, and 39. The factors of 55 are 1, 5, 11, and 55. The factors of 76 are 1, 2, 4, 19, 38, and 76. Thus, 11 is a prime number.